Don’t peak
When I talked to Michael Feathers about why constraints are good, he also mentioned an example of where he thought things were going wrong.
Some people who use test-driven development, also use tools that allow the to peek into classes and read private variables. That is an efficient way to ensure that some method altered the state of the object in the expected way.
However, as Michael pointed out, by doing so they miss the point of test-driven development. Test-driven development adds constraints. It does that in order to force developers to design their code better. (TDD is about design, not testing — remember?) By peeking into classes, one effectively removes one constraint added by TDD, and thereby removing much of the benefit of TDD.